In order to make primary aluminum by a conventional technique known as an electrolytic process, large carbon blocks are utilized as anodes. In order for these anodes to work efficiently and reliably, it is important that the carbon blocks have low electric resistivity and are free from internal flaws and cracks. Therefore, it is important for a primary aluminum producer to test the carbon anodes for internal flaws and low resistivity before they are used in the actual process for producing primary aluminum.
In conventional techniques, the primary aluminum producer must extract a core sample from a baked anode in order to perform the electric resistivity and internal flaw detection test. The core sample must be removed and sent to a laboratory in order for the measurements discussed above to be made. Once the results come back from the laboratory, these results are obsolete because the anodes which were produced concurrently with the core sample which was tested, have already been installed in an aluminum production pot line, and are either working fine or have already failed.
As a result, there exists a need for the primary aluminum producer to automatically and non-destructively test anodes in an in-line test setup for internal flaws and low resistivity, so that low quality anodes may be discarded before they fail in the production line. Several techniques, which have been proposed, are discussed below.
A first proposed technique for detecting the internal flaws in a carbon anode utilizes a change in DC resistance of the carbon block. At every contact point, the current enters the carbon block and spreads out into the carbon volume. Since the cross-section near the contact point, through which the current travels, is much smaller than further away, the total resistance of the current path is dominated by the resistance near the contact point. If the material has no random irregularities, this would not present a serious problem. However, in reality, the carbon blocks utilized as anodes in the electrolytic process contain thousands of such local irregularities which completely dominate the change in resistance. As a result, the change in DC resistance is not an accurate indication of internal flaws in the carbon anode. Still further, contact wear and the bridging of current and potential contact points by carbon dust are additional problems which may make internal flaw detection unfeasible by this approach.
A second proposed technique involves the use of ultrasonic sound to detect flaws in the carbon anode. However, the problems discussed above with respect to the DC resistance measurements are even more severe. In this technique, the signal reflected from flaws is used to detect cracks in the interior of the anode. Since the carbon block has thousands of irregularities which all produce back scattering, it is nearly impossible to distinguish random back scatter and backscatter from actual flaws. This distinction is even made more difficult because the random scattering attenuates the signal rapidly as it travels through the carbon block, so that random back scattering from a location close to the transducer can be much stronger than the back scattering from a serious flaw in the middle of the carbon block. In addition, this strong attenuation requires a large amount of energy to be coupled into the carbon block, which in turn produces even more random back scattering. As a result, the ultrasonic method is also unfeasible for carbon anodes.
As a result of the failures discussed above with respect to the DC resistance measurement and ultrasonic techniques, it is probable that any electrical measurement would have to be made such that the physical contact between the measuring device and the carbon block does not influence the measurement. Further, if sound waves are to be utilized, the energy coupling problem must be eliminated and scattering and attenuation must be drastically reduced.
One final technique which provides potentially promising results is an audio sound flaw detection method. For example, if two different carbon blocks are hit with a hammer, the sound generated by each is significantly different. Such an audio sound flaw detection system would eliminate the energy coupling problem present in the ultrasonic method and the much longer wavelength would reduce attenuation and back scatter. Further, preliminary measurements confirm that each carbon block appears to have its own distinct sound signature. This time domain signature can be converted into a frequency spectrum in order to reveal flaws in the carbon block. However, although it is a relatively simple task to convert the time domain signatures for each carbon block into frequency spectra, it is extremely difficult to determine which part of the spectrum represents flaws and which part illustrates the features of a good carbon anode. In order to successfully analyze the frequency spectra, this technique requires a homogeneous graphite block in order to calibrate the sound measurement instrumentation. Further, in order to ensure that the calibrations are free of environmental sound contamination, the calibration experiment would have to be conducted in an anechoic chamber, which is expensive.
The method and system of the present application solves the problems discussed above with respect to conventional carbon block analysis techniques, in that the method and system of the present application permit the primary aluminum producer to automatically and non-destructively test anodes in an in-line test setup, for internal flaws and high resistivity. Further, the method and system of the present application exhibit none of the problems discussed above with respect to the other conventional techniques. As a result, low quality anodes can be discarded at an early point in production.
The method and system of the present application utilizes two measurements in order to determine if a carbon block should be accepted or rejected for use in aluminum production. First, an eddy current loss measurement is made to detect the internal flaws of the anode, and second, a four-point resistivity measurement is made to measure the intrinsic resistivity of the anode. Together, these two measurements give an indication of the quality of the anodes.